For recovering gold and/or other precious metals from ores, a number of lixiviant systems have been proposed and used over the past century. The word "lixiviate" means to extract a constituent from a solid mixture. A lixiviant system is one that contains the components necessary to extract the desired constituent. The most widely used lixiviant system for gold is a combination of sodium cyanide as ligand together with air (oxygen) as oxidant. Hydrogen peroxide is sometimes used as an auxiliary oxidizing agent. Ores which are resistant to simple extraction or lixiviation procedures are commonly referred to as "refractory" ores.
Many gold-bearing deposits in rock were created by the precipitation of gold along with sulfide minerals during the flow of hydrothermal fluids through the rock. Depending on the deposition mechanism, the sulfide minerals can be present alongside the gold or can physically encapsulate it. Over time, the zone of such deposits nearest the earth's surface will have been oxidized by weathering, and the sulfides so oxidized carried away by groundwater flow. This zone is referred to as the "oxide zone". In the deepest portions of the deposits, below the water table, the sulfide minerals remain more or less in the form in which they were deposited. This zone is referred to as the "sulfide zone". The relative size of these zones is determined by the depth of the deposit, historical water table fluctuations and surface weathering conditions, among other factors.
Where the sulfide minerals persist in such a gold-bearing deposit, they demonstrate varying degrees of reactivity to sodium cyanide, the chemical lixiviant commonly used in gold leaching, and to oxygen, consuming them and requiring the addition of fresh materials. While some iron-containing sulfide minerals such as pyrite and chalcopyrite exhibit relatively low reactivity during the time span of most gold lixiviation processes, others such as pyrrhotite are highly reactive. The added processing cost due to consumption of lixiviant chemicals by a high concentration of these highly reactive minerals can make metal recovery from portions or all of a gold-bearing deposit uneconomic.
In the case of gold deposits where the gold is physically encapsulated in the sulfide minerals, the minerals can create a surface barrier which prevents the gold from being extracted. In this case, procedures such as roasting, pressure oxidation or biological oxidation of the deposit can be employed. Such procedures are very capital-intensive and costly.
In cases where the sulfide minerals do not physically block the access of the lixiviant solution to the gold, that is, the minerals are present with the gold but do not encapsulate it, an excess of lixiviant can be used, or the gold deposit can be pretreated in some way to passivate the surface of the sulfide minerals to make them less reactive to the lixiviant solution.
For example, the use of dissolved oxygen as a pretreatment step before cyanide leaching is described in "The Chemistry of Gold Extraction" by Marsden and House, pages 191-193 and 277, publisher Ellis Horwood, 1992. It is stated that, while this pretreatment can oxidize and/or passivate the surfaces of some of the more reactive, reagent-consuming, sulfides, it is often only Capable of partial oxidation of sulfides and is usually unsuitable for the treatment of ores where gold is intimately mixed with sulfides. It is further stated that "ores containing significant amounts of sulfides that cannot be passivated adequately by pre-aeration, and which result in unacceptable cyanide and/or oxygen consumption, must be treated by alternative processes, e.g. pressure oxidation, roasting."
Variations on this method of pretreatment include treatment of the ore with lime as well as air, or treatment of the ore with a chemical oxidizing system containing calcium hypochlorite. These methods are discussed in "Gold and Silver Cyanidation Plant Practices" by McQuiston and Shoemaker, pages 12-13, The American Institute of Mining, Metallurgical and Petroleum Engineers, 1975. These methods are often not completely effective in passivating the sulfide-containing ores.
U.S. Pat. No. 4,421,724 discloses a process for eliminating such pretreatments for precious metal recovery by modifying the cyanide extraction step itself. In this process, a commuted, refractory precious metal ore is agitationally treated with an aerated alkaline solution containing a high concentration of cyanide ion, 112 to 336 grams per gallon, and a low concentration of chemical oxidizer, 0.5 to 10 grams per gallon. The patent exemplifies potassium permanganate and other manganates as chemical oxidizers, and states that any suitable chemical oxidizer can be employed which can supply solubilized oxygen for enhancing the rate of precious metal solubilization and, at the same time, convert refractorizing constituents such as sulfides and arsenides into forms which are substantially inert to cyanide and air source oxygen. This process has the disadvantage of requiring unusually high cyanide concentrations.
U.S. Pat. No. 5,034,055 discloses a process for recovering gold and silver values from ore using activated carbon as adsorbent for the metal values wherein said activated carbon treated with an oxidant having an oxidation potential higher than that of oxygen. A preferred oxidant is potassium permanganate. The actual addition point of the oxidant may be before, after or during the leaching stage. Wherever added, its primary function is to treat the activated carbon so as to increase the recovery of silver. When the oxidant is added before the leaching stage, it also can function as an auxiliary oxidizing agent in the leaching solution before going on to treat the activated carbon. This patent does not disclose a method of reducing reagent consumption in a lixiviant system by pretreatment of the gold ore.
There is a need for an effective pretreatment for refractory gold-bearing deposits which contain iron-containing sulfidic minerals that do not encapsulate the gold, without excessive consumption of lixiviant chemicals. There is also a need for such pretreatments for other precious metal deposits which contain such sulfidic minerals.
Certain nickel and cobalt ores also contain iron-containing sulfidic minerals such as pyrrhotite, making the ores unsuitable for cyanide leaching. There is also a need for an effective pretreatment to make these ores cyanide-leachable.
A closely related problem, known as acid rock drainage, occurs in the case of iron-containing sulfidic materials resulting from mining and leaching of various metallic and non-metallic minerals. These sulfidic materials include, but are not limited to, tailings, overburden, discarded waste rock removed along with ore, and unmined exposed rock such as in pit walls. The natural air/water oxidation processes described previously in relation to the surface layers of a gold-bearing deposit (the oxide zone) will also occur with these materials, causing the formation of sulfuric or related acids. These acids are the cause of severe pollution problems throughout the world. Similar problems occur with the exposed surfaces resulting from coal mining.
Various attempts to correct these problems are described in U.S. Department of the Interior Bureau of Mines Publication SPO6B-94, covering the joint meeting of the International Land Reclamation and Mine Drainage Conference and the Third International Conference on the Abatement of Acidic Drainage held in April 1994. Attempts to render these sulfidic materials non-reactive included partially converting pyrrhotite into an oxide structure where each iron sulfide particle is coated with an iron oxide film, microencapsulation of pyrite by artificial inducement of FePO.sub.4 coatings, and (for underground coal mines), the coating of exposed surfaces with various polymeric materials. These methods of treating such materials have been at best only partially effective. Improved treatments are necessary to treat iron-containing sulfidic minerals to prevent or minimize the natural oxidation of these materials to form destructive acids.